This invention relates to support diaphragms for elements which are translated axially, especially for servo-focused optical elements in an optical disc recorder.
Optical disc recorder-playback arrangements (recorders) ordinarily use one or more laser diodes to produce beams of light which are focused onto the surface of a rotating disc for causing changes in the surface of the disc for recording, or for sensing the state of the disc for playback. In order to achieve a high stored data density on the disc, the individual recorded data bits must be represented by small physical features on the disc surface. At the current state of the art, each data bit as recorded on the storage disc has dimensions on the order of a wavelength of light.
In order to record or sense such small data imprints, the beam of light produced by the laser of the recorder must be focussed onto the disc surface. As the storage disc rotates, unavoidable manufacturing tolerances, together with the effects of imperfect aging of the disc materials, result in variations in the distance between the disc surface and the lens. These variations of distance may result in defocussing of the light beam at the disc surface, which in turn results in spreading of the data over a larger portion of the disc surface during recording, or in sensing of more than one data element or bit upon playback. In either case, the result may be inability to properly record and recover the data.
In order to prevent loss of data, optical disc recorders ordinarily provide a focus servo loop for sensing the amount of focusing at the disc surface, and for moving the lens assembly in a direction normal or orthogonal to the disc surface in order to attempt to maintain focus. For this purpose, the lens assembly is resiliently mounted to a frame (which may itself be mounted for radial translation relative to the disc), and arranged with transducers which apply forces to the lens assembly to urge it in the desired direction under the influence of the servo loop. As is known to those skilled in the servo art, such servo loops have characteristic gain-bandwidth curves which are established by the characteristics of the elements of the servo loop, which includes the effects of the characteristics of the lens assembly and its resilient mounting. Very often, the major limitations on the gain or bandwidth of the servo loop are imposed by the mass and resonant characteristics of the movable portions of the lens assembly and of the resilient mounting. Since the rate of variation of distance is determined by the angular velocity of the disc, the speed of focus may be the salient limitation on disc speed, which in turn is a major limitation on data rate, both in recording and in reading data.
Optical discs often make 30 or 60 rotations per second (RPS). The variations in spacing between the disc surface and the lens which occur at the corresponding once-around rate of 30 Hz or 60 Hz are caused by disc warpage, and these tend to be the largest-amplitude spacing variations. Ordinarily, discs have less than 0.010 inch (0.25 mm) of warp. The surface of the disc is irregular at a microscopic scale, and the rate at which the spacing variations which correspond to the microscopic irregularities pass the read head may be represented as an amplitude-frequency spectrum which resembles a noise spectrum, in which the larger variations tend to occur at lower frequencies, with the higher-frequency variations having a lower amplitude. Thus, the largest amplitude variations of a typical disc may be accommodated by a focus head movement of about 10/1000 inch.
In the past, focus servos for optical disc recorders have had a frequency bandwidth of up to 4 kHz. However, the bandwidth of the significant high-frequency variations in the disc-to-lens assembly spacing extends to higher frequencies. It is known that the mass of the lens assembly must be minimized and its resonant frequency maximized, and that the resilient mounting must provide smooth axial translation, a range of compliance at least equal to the largest expected spacing variations, and a high resonant frequency.
In the prior art, a resilient mounting has been provided by one or more spiral springs wound about a substantially cylindrical lens assembly, with one end of each spring attached to the lens assembly and the other end attached to a frame. As described below, such a structure tends to have relatively low frequency non-axial modes of vibration and particularly vibration of the spring itself which limit the focus servo frequency bandwidth.
A resilient mounting for a lens assembly is desired which provides a large range of smooth compliance, and for which undesirable modes of vibration are at a high frequency.